1. Field
Aspects of the present invention generally relate to an image forming apparatus including a function of detecting a surface potential of an image bearing member on which a latent image is formed.
2. Description of the Related Art
An electrophotographic image forming apparatus includes a photosensitive drum serving as an image bearing member. The photosensitive drum is used when the image forming apparatus forms an electrostatic latent image and develops the formed electrostatic latent image with toner to form a developer image (image). When the electrostatic latent image is formed on the photosensitive drum, a surface of the photosensitive drum needs to be charged. A charge amount necessary to charge the surface of the photosensitive drum is changed depending on various factors such as environment in which the image forming apparatus is used, a film thickness and sensitivity of the photosensitive drum, and a variation of circuit elements in, for example, a high-voltage circuit used for a charging operation. A change in the charge amount causes a variation in a potential difference (also called a back contrast) between the charge amount of the photosensitive drum and a development voltage (also called a development bias) used to develop the latent image with toner. Such a variation may degrade image quality or affect a toner consumption amount.
Moreover, when the photosensitive drum has been used for a long time, a surface layer of the photosensitive drum becomes abraded. This significantly decreases a charge amount, causing degradation in image quality including generation of a defective image and a decrease in image density.
Japanese Patent Application Laid-Open No. 2000-347545 discusses a method for suppressing image quality degradation due to deterioration in the photosensitive drum. According to this method, a surface potential of the photosensitive drum is detected by a surface potential measuring unit, and an image forming condition is controlled according to the detection result.
However, the method using the surface potential measuring unit discussed in Japanese Patent Application Laid-Open No. 2000-347545 can only detect a local potential on the photosensitive drum. Consequently, in a case where the surface potential measuring unit detects a surface potential of the photosensitive drum with dust and toner attached to the surface of the photosensitive drum, the surface potential cannot be accurately detected. Thus, it has been desired that the surface potential of the photosensitive drum is detected with accuracy regardless of the surface state of the photosensitive drum.